This invention relates to the heating of print media that is advanced through an ink-jet printer.
An ink-jet printer includes at least one print cartridge that contains liquid ink within a reservoir. The reservoir is connected to a print head that is mounted to the body of the cartridge. The print head is controlled for ejecting minute droplets of ink from the print head to a print medium, such as paper, that is advanced through the printer.
Many ink-jet printers include a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the droplets onto the paper is controlled to form a swath of an image with each scan. Between carriage scans, the paper is advanced so that the next swath of the image may be printed.
Oftentimes, especially for color images, the carriage is scanned more than once across the same swath. With each such scan, a different combination of colors or droplet patterns may be printed until the complete swath of the image is formed. One reason for this multi-scan print mode is to enable the ink of one color to dry on the media before printing a second color pattern that abuts the first pattern. This print mode thus prevents color bleeding that might otherwise occur if two abutting, different-colored droplets were printed at the same time.
The speed with which the print media is moved through a printer is an important design consideration, called xe2x80x9cthroughput.xe2x80x9d Throughput is usually measured in the number of sheets of print media moved through the printer each minute. A high throughput is desirable. A printer designer, however, may not merely increase throughput without considering the effect of the increase on other print quality factors.
For instance, one important factor affecting the print quality of ink-jet printers is drying time. The print media movement must be controlled to ensure that the liquid ink dries properly once printed. If, for example, sheets of printed media are allowed to contact one another before ink is adequately dried, smearing can occur as a result of that contact. Thus, the throughput of a printer may be limited to avoid contact until the sheets are sufficiently dry. This potential for smearing is present irrespective of whether ink is applied by a scanning technique as discussed above or by other methods, such as stationary print head arrangements that effectively cover an entire width of the print media.
Scanning type ink-jet printers must have their throughput controlled so that separate scans of the carriage are spaced in time by an amount sufficient to ensure that no color bleeding occurs as mentioned above.
In addition to throughput, an ink-jet printer designer must be concerned with the problem of cockle. Cockle is the term used to designate the uncontrolled, localized warping of absorbent print media (such as paper) that occurs as the liquid ink saturates the fibers of the paper, causing the fibers to swell. The uncontrolled warping causes the paper to move toward or away from the print head, changing both the distance and angle between the print head and the paper. These unpredictable variations in distance and angle reduce print quality. A predictable and constant distance and angle are desired to assure high print quality. Even if the occurrence of cockle does not affect this aspect of print quality, the resultant appearance of wrinkled print media is undesirable.
Heat may be applied to the print media in order to speed the drying time of the ink. Heat must be applied carefully, however, to avoid the introduction of other problems. For example, if the heat is not uniformly applied to the printed media, the resultant uneven drying time of a colored area of an image can produce undesirable variations in the color""s hue characteristic.
Another problem attributable to improperly applied heat can be referred to as xe2x80x9cbuckling.xe2x80x9d Normally, print media carries at least some moisture with it. For example, a sealed ream of standard office paper comprises about four and one-half percent moisture. High amounts of moisture in the media, such as paper, may be present in humid environments. As heat is applied to part of the paper, uneven drying and shrinkage occurs. The uneven shrinkage causes the paper to buckle in places, which undesirably varies the distance between the paper and the print head, as occurs with the cockle problem mentioned above.
Some print media, such as polyester-based transparency print media, will carry insignificant amounts of water and, therefore, will not buckle as a result of uneven shrinkage. Such media, however, may buckle if all or portions of it are overheated. Thus, uniform, controlled heating of the media is important for high print quality, irrespective of the type of print media.
If heat is applied to the media, it is useful to have it applied in the print zone of the printer. The print zone is the space in the printer where the ink is moved from the print head to the print media. Thus, the media is moved through the print zone during a printing operation. Heating the media in the print zone rapidly drives off (evaporates) a good portion of the liquid component of the ink so that cockle is unable to form, or at least is minimized, and so that the time between successive scans of the same swath can be minimized.
When one attempts to heat the media in the print zone, it is important to ensure that the applied heat is not directed to the print head of the cartridge. If the print head overheats, droplet trajectory and other characteristics of the print head can change, which reduces print quality. Also, the heat should not be applied in a way (as by convection) that may directly alter the droplet trajectory. The heat should be applied in a cost-efficient manner.
Another printer design consideration involves the support of media in the printer for precise relative positioning and movement relative to the print head of the cartridge. Vacuum pressure may be used to support print media for rapid advancement through the printer. One method of supporting a sheet of print media is to direct it against an outside surface of a moving carrier such as a perforated drum or porous belt. Vacuum pressure is applied to the interior of the carrier for holding the sheet against the moving carrier. The carrier is arranged to move the sheet through the print zone.
The vacuum pressure or suction (Here the term xe2x80x9cvacuumxe2x80x9d is used in the sense of a pressure less than ambient, although not an absolute vacuum.) must be applied at a level sufficient for ensuring that the sheet of print media remains in contact with the carrier. Moreover, a uniform application of vacuum pressure to the media will help to eliminate the occurrence of cockle in the sheet because the vacuum pressure helps overcome the tendency of the media fibers to warp away from the surface of the carrier that supports the media.
With the foregoing in mind, the present invention may be generally considered as a technique for heating print media in an ink-jet printer. As one aspect of this invention, heat is uniformly applied to the media in conjunction with mechanisms for uniformly applying vacuum pressure to the media for supporting the media as it moves through the printer.
The heat is efficiently applied to the media by conduction, in a manner that will not overheat the print cartridge print head nor interfere with the trajectory of the droplets expelled from the print head. The hardware for applying the heat has high thermal transfer efficiency and low thermal mass. As a result, there is less likelihood of overheating the print cartridge or other printer components through heat radiation from the heating components after the paper is moved from the print zone.
In a preferred embodiment, the heat is applied to the media in the print zone as well as regions on either side of the print zone, where the media respectively enters and exits the print zone. The entry region is sized and heated by an amount that ensures that media is sufficiently dry before entering the print zone so that shrinkage and buckling does not occur in the print zone, thus ensuring that a constant distance and angle is maintained between the media and the print head.
The amount of heat applied to each of the entry and exit regions and to the print zone is independently controlled. The amount of heat applied can be related to the physical characteristics of the particular type of print media or inks that are used, or the ink densities of the image being printed. Also, the thermal transfer efficiency of the heater mechanisms provides a quick temperature rise time so that the paper can be heated quickly, thus permitting high throughput.
Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.